Although the blast generated in the underground test was large enough to trigger a mini earthquake, experts say a hydrogen bomb - or "H-bomb" - would have generation one many times as great.

"The bang they should have gotten would have been 10 times greater than what they're claiming," Bruce Bennett, an analyst with the Rand Corporation, told the BBC.

"So Kim Jong-un is either lying, saying they did a hydrogen test when they didn't, they just used a little bit more efficient fission weapon - or the hydrogen part of the test really didn't work very well or the fission part didn't work very well."

Until now, Pyongyang was known to possess only more basic atomic bombs - of the kind dropped on Hiroshima and Nagasaki during World War Two. Such bombs work via fission, or the splitting of atoms.

Hydrogen bombs are far more sophisticated and destructive, however, and use a process known as "nuclear fusion" to merge atoms and generate an enormous burst of energy.

Chinese military expert Du Wenlong echoed Bennett's skepticism in an interview with state TV, saying the evidence "doesn't support suggestions that the bomb was a hydrogen bomb."

And while South Korea denounced news of the test as "a grave provocation," South Korean intelligence officials apparently believe the blast is too small to have been an H-bomb.

The UN Security Council is set to hold an emergency meeting on the topic later Wednesday.